The present invention relates to an optical information recording/reproducing apparatus, and more particularly to an information recording/reproducing apparatus for achieving a high recording density.
A method for increasing the recording density for an optical information recording/reproducing apparatus has conventionally been accomplished by improving a recording plane density on a two-dimensional plane of a recording medium. However, since the size of information recording media, such as a disc, is restricted due to compactization of the apparatus, two-dimensional approach to high density will reach its limit. As a method of achieving a higher density, there has been proposed a three-dimensional recording/reproducing method which is related with the record in the depth direction of a recording medium.
For example, the JP-A-59-127237 discloses that information is recorded on two recording layers by using two lights of different wavelengths. In this event, when a light irradiates a recording layer on the incident plane side through the other recording layer, energy of the light is absorbed in the recording layer on the incident plane side, whereby information is unintentionally recorded thereon. Therefore, in JP-A-59-127237, writing on the other recording layer is only allowed after information has been recorded on the recording layer on the incident side. More specifically, following three-value informations can be recorded for the states where information is recorded on both two recording layers; no information is recorded on both recording layers; and information is recorded only on the recording layer on the incident plane side.
However, the JP-A-59-127237 does not disclose a method of independently recording binary information on each of multi-layer recording film.
Also, the JP-A-60-202545 discloses a method of focusing a laser beam on each layer of the multi-layer recording films. Generally, a focusing servo circuit focuses a laser beam on a recording film by supplying an electric offset when the beam is out of focus. By utilizing this method, offset voltages corresponding to respective inter-layer gaps in the multi-layer recording film have previously been prepared. Then, one of the offset voltages corresponding to a layer to be focused is supplied to focus the laser beam on that layer.
However, the JP-A-60-202545 does not disclose any means for corresponding the multi-layer recording film to the offset voltages.
Also, the JP-A-60-202554 discloses a multilayer recording medium in which inter-layer gaps are formed equal or larger than an operating range of a focus error signal by more than the same operating range. However, the JP-A-60-202554 does not describe specific inter-layer gaps and an access method of actually focusing a beam on a target layer.